The present invention relates to a process for drying sawmill timber and wood items.
The invention is particularly useful for the treatment of xe2x80x9csawmill timberxe2x80x9d. By the wording xe2x80x9csawmill timberxe2x80x9d is meant timber directly derived from initial processing (sawing).
A process is known from patent application WO 82/01766 for drying timber which uses microwaves at a frequency of 915 MHz applied to wood to be dried in order to raise its inner temperature and cause it to discharge water. The discharged water is evaporated on the surface of the wood by a circulation of air at low speed obtained using fans. The air with a moisture content of approximately 89% passes over condensers to extract this moisture.
In document WO 82/01411, the same principle is used, but in this latter document it is specified in addition that the temperature of the air must always be lower than the inner temperature of the wood. This document points out the drawback of having to heat the surface area of the material with microwaves before heating the inner part of the material. In this document, it is therefore proposed to control the process of converting magnetic energy into heat energy so as to concentrate the waves on the water within the material. Also, it is suggested to act on the climate within the chamber by maintaining a sufficiently high percentage of air humidity to prevent the surface of the product from drying out before removal of the moisture from the core of the wood. For this purpose, during the initial phase of the drying process, water in atomized form is added to the chamber to maintain a high humidity level.
Similarly, the article published in the review xe2x80x9cHolz als roh und werkstoffxe2x80x9d in 1995, pages 333 to 338, Springer-Verlag Editions, entitled xe2x80x9cMicrowave drying of pine and sprucexe2x80x9d by A. L. ANTTI describes drying wood with microwaves operating at frequencies of 915 or 2450 MHZ and a power density in the range of 25 to 78 kW/m3 to raise the inner temperature of the wood to approximately 140xc2x0 C. and achieve a steam pressure inside the wood of 25 KPa. The inner pressure achieved in this way is very high and enables swift evacuation of the water. The disadvantage of the process is that it develops breaks in fibres. The drying process starts by quick microwave drying in the region of 70xc2x0 C. followed by intermittent exposure to microwaves during drying, and finally a drying operation under wood temperature control to remain below fibre saturation by limiting temperature to a maximum of 110xc2x0 C.
In all configurations, it is evident that air is used as the vehicle to remove the moisture which exits from the wood. On this account, the air humidity level must remain below the saturation level of air in steam. It is therefore necessary in known systems to de-humidify the air in order to carry out wood drying. Also, it is necessary for the air temperature to be lower than the wood temperature to allow evaporation. All these systems have the disadvantage of generating large energy losses and do not optimise energy consumption. The higher the required wood temperatures, the greater the proportional amount of microwave generating power is required, and since drying times last several hours energy consumption is high and therefore costly. It can be noted in the article cited above that drying times are between 3 and 5 hours depending upon wood thickness and the power of the equipment used. Also, none of these known processes manages to achieve less than 30% moisture content in the wood after drying.
The purpose of the present invention is to put forward a process with which it is possible to optimise energy and reduce the power of the microwave means while rapidly obtaining complete drying of the wood, from the green state to a final moisture content in the order of 10%, or even less depending upon operating conditions.
This purpose is achieved through the fact that the wood drying process comprises:
a pressurizing step, to place at least one sealed chamber 1 under a determined pressure by injecting or creating saturating steam, and maintaining this pressure for a determined time interval while ensuring forced circulation of air and saturating steam within the chamber;
a heating step, to heat the wood core and the central part of the wood items to be dried, by emitting microwaves at frequencies of between 400 and 2450 MHz;
an evacuation step, to carry away the liquid exudates which exit the wood and run down to the bottom of the chamber 1 where they are collected.
According to another particularity, the liquid exudates are permanently evacuated.
According to another particularity, the exudates are intermittently evacuated.
According to another particularity, the evacuation step is followed by a gradual pressure-lowering step down to atmospheric pressure after stoppage of the microwaves.
According to another particularity, the evacuation step comprises physico-chemical treatment of the exudates to make them compatible with evacuation towards the waste water circuit.
According to a further particularity, the liquid exudate evacuation step is followed by a collection step in a container for the purpose of further chemical treatment.
According to another particularity, the pressure-lowering step is completed by a de-humidifying step of ambient air in the chamber by passage of the stream of air from the chamber onto a humidity absorption device and cooling of the air within the chamber.
According to another particularity, the applied microwave emitting powers are of decreasing magnitude from the core of the wood pieces towards the outside.
According to another particularity, the saturating steam pressure is in the range of 2 bars to 15 bars.
According to another particularity, the steam pressure is less than 10 bars to obtain a treated wood moisture content of more than 6%.
According to a further particularity, the steam pressure, at least during a determined drying time, is between 10 and 15 bars and the temperature produced will reach a value lying in the range of 200 to 220xc2x0 C. to obtain a dry, naturally polymerized wood having a moisture content close to 0%.
According to another particularity, the power of the microwave generator is calculated so that the internal heat of the wood is higher than the temperature of the saturating steam.
A further purpose of the invention is a system enabling the implementation of the process.
This purpose is achieved through the fact that the system is made up of a pressure-resistant sealed chamber communicating via windows in quartz, or any other material suitable for microwaves, with a waveguide that is connected by impedance adapters to a microwave generator, said windows being arranged crosswise to the stack of wood, the chamber being connected to a pressurized air recirculation pathway which aspirates air from one side of the wood stack via grids and repels the air on the other side of the wood stack by means of diffusion grids, and pressurized steam generating means connected to the chamber.
According to another particularity, the system comprises a steam condenser circuit connected in parallel to the air recirculation circuit and in selective manner via valves.
According to another particularity, the system comprises in its lowest part an evacuation outlet operating under gravity to evacuate the exuded waters which is controlled by a valve.
According to another particularity, the system comprises:
one end which can be closed by an automatic door to ensure sealing against pressure and microwaves;
conveying means to transport the green wood loads to be dried, which means are electrically separated from the transport means located on the other side of the automatic airlock in relation to the chamber.
According to another particularity, the unit formed by the chamber and pre-loading zone is encased in a second protective chamber protecting against radiation leakage, this chamber being accessible from the outside via flexible doors.
According to another particularity, the microwave generator is embedded in the ground and communicates with the drying chamber via a waveguide.
According to another particularity, the chamber comprises a safety valve.
According to another particularity, the valve is opened intermittently.
According to another particularity, the valve is opened permanently.
According to a further particularity, the outlet is connected to a physico-chemical treatment system to render the exudates compatible with waste water evacuation standards.
A final purpose of the invention is to put forward a chemical component extraction method using the process and system of the invention consisting of:
treating one single type of green wood species by applying microwaves in an atmosphere of saturating steam under determined pressure and temperature conditions,
collecting the liquid exudate produced by the single species treatment operation,
optionally re-treating this exudate with physico-chemical methods to remove various chemical components that can be used in the cosmetic, perfume, agro-foodstuffs, pharmaceutical and chemical industries.
According to another particularity, the treatment of pine species leads to obtaining an exudate having insecticide properties.